Lecture Outline Nov18

Name ______________________________________ In section 11.2 of the text, addition of hydrogen halides to alkenes is discussed. There are two light blue boxes. One says "Markovnikov's rule" — in 1870, the mechanistic basis for reactivity was not understood, so applying this rule was the only way to predict the course of the addition of HX to an alkene. The problem is that the molecules never learned this rule, and today we know how to think like a molecule, i.e. to predict the course of reactions based on mechanistic ideas. The other box says "Mechanism-based rule". While the author clearly favors making predictions based on the mechanism, that point isn't made strongly enough, and having both blue boxes there almost makes it seem like we can pick whichever "rule" we like best. Baaaah. (Or as they said in 1870, "balderdash.") That the course of these reactions is governed by carbocation stability is not a "rule", it's a mechanistic principle. 1 In the reaction of each alkene below with H–X, (a) first draw the one most stable carbocation intermediate that can be formed by protonation of the π-bond — use what you already know about carbocations (no "rules" needed) — then (b) draw the product formed by addition of the nucleophilic halide ion. Include stereochemistry. H–Br H–Cl Ph 2 Same as #1, but take a careful look at the carbocation intermediate — hmmm.... would that perhaps do something other than just hang out waiting for the nucleophile? (What would Markovnikov's rule say about this?) H–I Lecture outline The most common/useful reactions of alkenes and alkynes are additions to a π-bond. 1. a. Electrophilic addition of H–X (X = Cl, Br, I) H C C + HX X C C This reaction is regiospecific — reaction with an unsymmetrically substituted alkene produces only one of two possible structural isomers CH3 H2C C CH3 CH3 HBr H2C CH3 H C Br CH3 only product H2C C Br H CH3 none (Regiochem is said to follow "Markovnikov's rule") The reaction mechanism accounts for the observed regiochemistry (and more) Complete the mechanism by adding the intermediate structures and the curved arrows. HBr Br H Br 1° cation 3° cation — the rate-limiting first step is an "electrophilic addition", i.e. addition of an H+. The reaction goes via the most stable carbocation only. Because the transition state for the rate limiting first step resembles the cation, the pathway to the more stable cation has the lower activation energy. sketch rxn coordinate diagrams for the two competing rxn pathways The difference in cation stabilities is about _____ kcal/mol Therefore the difference in Eas is also about _____ kcal/mol. This means that the route via the more stable 3° cation is about ______ times as fast as the route via the less stable 1° cation A few more examples. Let's include stereochemistry. HCl CH3 CH2 CH H–I CH2 H–I HBr b. Acid-catalyzed hydration — addition of "H" and "OH" — Markovnikov regiochem OH H+ H2O Where's the H that added? Do you see that H? Look carefully! (or "H 3O+") mech: This is just the reverse of the acid-catalyzed dehydration — the principle of microscopic reversibility demands that the mechanistic pathways be the same — why must this be true? With the mechanism in mind, draw the products of these reactions H2SO4 H2O H+ H2O c. Alcohols react just like water. Why is this not surprising? Write the mechanism. H+ R–OH O–R 2. Electrophilic addition of halogen ("X+") a. Halogenation — addition of two halogen atoms X C C + X—X C C X X = Cl, Br (X ! I !G° > 0) — this rxn is often carried out in CH2Cl2, CCl4, or another inert (non-reactive) solvent How does this happen? Let's first think about what we would expect if Br2 were to add to an alkene the same way HBr does, i.e., via a simple carbocation intermediate... C C Br—Br C C Br Br C Br C Br If this mechanism is correct, (1) what stereochemical outcome would you expect in this case? Br2 (2) What product would you expect in this case? Cl2 What really happens? mechanism: Br Br C Br –Br C C Br C Br2 is polarized by !-electrons C Br C C C resonance structures !+ actual structure: Br ! + C C Bromonium ion !+ Because the positive charge is delocalized, bromonium ions are relatively stable and have no tendency to rearrange. Bromide ion opens up the bromonium ion by a backside SN2 attack, just like we saw for protonated epoxides. frontside attack — verboten! Br Br C Br C C C Br backside SN2 attack